In the conventional optical scanner, a single lens with a specific resolving power is usually designed in accordance with a user's requirement; a scanner with a high resolving power is usually used to input drawings or characters, while a scanner with a low resolving power is used to input simple documents or papers. Theoretically, the scanner with a high resolving power would provide a better scanning quality; however it has a lower scanning speed as a result of having to scan more spots. Therefore, a user may select a scanner with faster scanning speed and with an acceptable resolving power in the same time, if possible.
Currently, the lens used within an optical scanning device is usually designed and limited to a single lens with one resolving power (as shown in FIG. 1); when a user has to scan various kinds of papers for input processing, he has to purchase two or more different scanning machines which costs more, or to tolerate the poor scanning quality or the slow scanning speed; in other words, the aforesaid conventional scanner has many inconveniences and drawbacks in the lens mechanism thereof, and an improvement therefor is required.